[mothered]

Since the earbuds and watch were in the same package, and on the package it said it was 0.5kg, how could they say the weight was correct upon delivery?

That's precisely my point In my previous posts.

since the package was less than 500 grams, it gets rounded up to 0.5kg. If I just ordered the watch on its own, which was 213 grams, the package would have still said 0.5kg because it's still within that 0 to 500 gram range.

It doesn't work that way. Shipping scales do not Increment at 500 grams.

For Instance, the watch was detected at 213 grams, so they'd deduct that figure from the package weight at 500 grams. If the watch was missing, the package should've weighed 287 grams (500 - 213 = 287) but It didn't. That's why they've said the weight was correct.

Couldn't I say that someone must have taken the watch out and replaced it with carrdboard instead? and that the weight of the package doesn't reflect the actual contents of it?

Don't make suggestions.

Simply say you've noticed signs of tampering.